I tracked down the install instructions for the KW V3 for the S197 and it specifically shows a picture of re-using the rear "spring support" (except that it's misspelled in the KW document) at the top of the spring. If that's rubber, then you've got as much isolation as the stock setup.
Did you change anything else? Panhard, LCA's, UCA? Also check the damper install. The rear dampers are supposed to center in the middle of an upper/lower pair of rubber bushings, and the torque spec is surprisingly soft. If it isn't quite lined up right and the shafts are touching the body or if the nuts are over-torqued, you might get the same problem of noise transmission. However, it's pretty obvious how they go together so unless your shop was really asleep at the switch, it's a long shot.
This, by the way, is one of the big reasons I do my own work. It might take me all day to do a two-hour job, but when I'm done I know exactly how it's put together. If I'm not happy, I have a starting point for changes.
Originally Posted by Ryan Hill
Well, I just got off the horn with KW and they told me that there's no reason there should be any noise being transferred into the cabin of the car, especially if using the stock top-hat, which I am.
I'm taking it back to JoTech and I'm going to have them tear down the whole thing if that's what it takes.